Omnidirectional circularly polarized broadcast antennas for stations operating in the FM band are often of the type described in previous U.S. Pat. No. 3,474,452 of Richard D. Bogner, one of the applicants herein. That is, each bay consists of thin arms (less than 1/20 wavelength in girth for at least a substantial part of their length) extending away from a vertical coaxial transmission line, and are excited from the line by one (or more) thin metal straps connected between the line center conductor and one arm. These designs have the characteristics of being narrow band i.e. the impedance varies rapidly with frequency such that the VSWR is often under 1.1 over only 50 to 100 KHz, whereas the station transmits a 200 KHz band. The antennas are critical with regard to the region of the strap and the small opening in the vertical coaxial line, such that dirt, ice, humidity, very small mechanical movements tend to detune the antenna. This prior art construction frequently requires the use of fine tuners, radomes, deicers and frequently require repair or readjustment. The excitation straps and the narrow spacing also makes possible voltage breakdown in that area of the antenna.
It has been found that these undesirable characteristics can be eliminated, while maintaining or improving the generally desirable radiation pattern and axial ratio characteristics of this antenna type. This is achieved by employing slot excitation and "fat" arms haing a girth of at least 0.1.lambda. or more throughout substantially their entire length of about .lambda./4 each. Thus there is eliminated the small opening in the coaxial outer conductor and the strap connected through it (to the center conductor on one end, and to a point on one arm on the other end). Arm excitation is achieved by providing a long slot on both sides of the support tube, the slot extending about .lambda./4 on each side of the feed point, the width of the band having a VSWR under 1.1 can be increased to as much as 1 MHz, the pattern made perfectly symmetrical, and effects of dirt, ice, power, humidity, tolerances and movement made negligible so that no additional radome, deicer, or tuner is needed. It is, in general, desirable for structural reasons due to the thin support tube wall often employed and also to allow line pressurization, to cover the slots with an electrically non-conductive but stong material, such as resin impregnated glass fibers. The arms may be e.g. plates or tubes and, in general, thrust or spiral outwardly and upwardly on one side and out in the same direction and downwardly on the other side of the slot.
In the aforementioned application there is disclosed an antenna comprising a coaxial transmission line provided with a pair of opposed slots in the outer conductor forming a balun to feed a pair of dipoles.
There is a need to enclose the open slot in a pressure tight arrangement to permit of pressurizing the line to prevent the entrance of moisture which would cause failure of the antenna. The construction taught in the said copending application while operative is costly to manufacture and not susceptible to be repaired.
The present invention utilizes a rigid sleeve of filament wound fiberglass secured to bushings bonded to the transmission line. The bushings are provided with a O-ring groove containing an O-ring which seals to the sleeve.